Interim Standard IS-95-A (IS-95) has been adopted by the Telecommunications Industry Association for implementing CDMA in a cellular system. In the CDMA system, a mobile station communicates with any one or more of a plurality of base stations dispersed in a geographic region. Each base station continuously transmits a pilot channel signal having the same spreading code but with a different code phase offset. Phase offset allows the pilot signals to be distinguished from one another, which in turn allows the base stations to be distinguished. Hereinafter, a pilot signal of a base station will be simply referred to as a pilot. The mobile station monitors the pilots and measures the received energy of the pilots.
When in a call, the base station communicates with the mobile station over Forward Traffic Channels. During the call, the mobile station must constantly monitor and maintain four sets of pilots (collectively referred to as Pilot Set)--the Active Set, the Candidate Set, the Neighbor Set, and the Remaining Set. The Active Set are pilots associated with the Forward Traffic Channels assigned to the mobile station. The Candidate Set are pilots that are not currently in the Active Set but have been received by a particular mobile station with sufficient strength to indicate that the associated Forward Traffic Channels could be successfully demodulated. The Neighbor Set are pilots that are not currently in the Active Set or Candidate Set but are likely candidates for handoff. The Remaining Set are all possible pilots in the current system on the current CDMA frequency assignment, excluding the pilots in the Neighbor Set, the Candidate Set, and the Active Set.
As the mobile station moves from the region covered by one base station to another, the mobile station promotes certain pilots from the Neighbor Set to the Candidate Set, and certain pilots of the Candidate Set are subsequently promoted to the Active Set by the base station. The mobile station notifies the base station or base stations of the promotion from the Neighbor Set to the Candidate Set via a Pilot Strength Measurement Message. The base station notifies the mobile station of the promotion from the Candidate Set to the Active Set via a Handoff Direction Message. The process of continually updating the Pilot Set during a call is referred to as "pilot set maintenance."
A conventional algorithm for determining which pilots to promote to the Candidate Set repeatedly performs the following steps in the order stated as follows: 1) scan all active pilots; 2) scan all candidate pilots; 3) scan one neighbor pilot, or one remaining pilot when the entire Neighbor Set has been scanned. When the measured energy of a neighbor pilot or a remaining pilot is found to be above the a certain threshold T.sub.-- ADD, the pilot is promoted to the Candidate Set.
A single energy measurement of a pilot is not as very reliable in the presence of Rayleigh fading, and some neighbor or remaining pilots may be promoted when the energy of the neighbor pilot is not above T.sub.-- ADD. To compensate, a variation of this algorithm specifies a fifth set, called a Pre-candidate Set. The purpose of the Pre-candidate Set is to ensure that the energy of a pilot is above T.sub.-- ADD before being promoted to the Candidate Set.
The Pre-candidate Set is scanned along with the Candidate Set in the order stated above. When the energy of a neighbor pilot is first found to be above T.sub.-- ADD, the neighbor pilot is promoted to the Pre-candidate Set instead of directly to the Candidate Set. The base station is not informed about the pre-candidate pilot until it is promoted from the Pre-candidate Set to the Candidate Set.
A pre-candidate pilot is determined to be promotable to the Candidate Set by means of a simple state machine. A pilot promoted to the Pre-candidate Set starts in state 2. Each time the pre-candidate pilot (i.e., the recently promoted pilot) is scanned and the measured energy is above T.sub.-- ADD, the state of the pre-candidate pilot is incremented. When the pre-candidate pilot is scanned and its measured energy is below T.sub.-- ADD, its state is decremented. When the corresponding state reaches 5, the pre-candidate pilot is promoted to the Candidate Set. When the corresponding state reaches 0, the pre-candidate pilot is demoted to the Neighbor Set.
IS-95 specifies that the mobile station shall support an Active set of size 6, a Candidate Set of size 5, and a Neighbor Set of size 20. These are the minimum set sizes that must be supported. When there are several active, candidate, and pre-candidate pilots in their respective sets, neighbor pilots are scanned relatively infrequently. For example, when there is a total of 3 active and candidate pilots and 20 neighbor pilots, the algorithm would result in more than 80 scans between scans of the same neighbor pilot. At 15 milliseconds (ms) per scan, this results in 1.2 seconds between scans of the same neighbor pilot. This is unacceptable because IS-98, the mobile station performance specification for CDMA, specifies that the mobile station must promote an acceptable pilot from the Neighbor Set to the Candidate Set in no more than 800 ms.
The mobile station must quickly determine which neighbor pilots to promote to the Candidate Set to ensure that the strongest pilots will remain in the Active Set. For example, in a rapidly changing environment, such as when the mobile station goes around the corner of a large building, the active pilots may be rapidly shadowed, and the measured energy of a neighbor pilot may change from very weak to very strong. If the mobile station is not able to promote the new strong neighbor to the Candidate Set quickly enough, i.e., before communication on the active pilots is lost, then the call will "drop." This phenomenon has been termed "Rapid Changing Pilot" problem.
A need therefore exists for a method of and apparatus for significantly improving the mobile station's performance in a rapidly changing environment by maintaining the Pilot Set in such a manner that the strong pilots are quickly determined and promoted to the Candidate Set.